Plug the iPad into computer and open up iTunes (You can download iTunes by clicking here).

Turn off iPad while it is plugged into the computer. Once it is off turn the iPad on but hold the home button while doing so until it shows the iTunes recovery logo.

Then iTunes will tell you that it has detected an iPad in recovery mode and needs to either be restored or updated.

Select the restore option as restore is the only way to remove a passcode off an iPad which is unfortunate as it wipes all data on the iPad as well.

iTunes will then download the iOS system software which will take some time. On good internet it will take 5-15 minutes on average.

After 5 minutes the iPad will most likely kick itself out of recovery mode (recovery mode idle timeout). If this happens you will need to get the iPad into recovery mode again. Use the same steps mentioned above to boot the iPad into recovery mode again.

If the iPad has activation lock enabled on it, you will need to enter in your Apple ID and password linked to the iPad to activate the iPad which will prompt you during the initial setup process after restoring an iPad.

If you do not remember or know the linked Apple ID with the password then you will either need to reset password via email forgotten password on apple's website or get it removed via apple if you are the original owner of the iPad with proof of purchase.